Method of annealing steel coils moving through a furnace

ABSTRACT

A FURNACE FOR ANNEALING WHOLE COILS OF SILICON STEEL WHICH INCLUDES A WELDED GAS TIGHT SHELL LINED WITH VARYING THICKNESS REFRACTORY MATERIAL DEFINING A PLURALITY OF INDEPENDENTLY CONTROLLABLE HEATING AND COOLING ZONES AND IN WHICH A HYDROGEN ATMOSPHERE IS MAINTAINED; THE FURNACE HAVING CARS OF SPECIAL DESIGN FOR TRANSPORTING THE COILS THERETHROUGH, SPECIAL HEATING ELEMENTS EXTENDING ALONG THE FURNACE WALLS OF THE HEATING ZONE AND A SUPPLEMENTAL COOLING ARRANGEMENT FOR THE COILS TO PROVIDE A CONTROLLED HEAT TREATMENT.

P 1973 w. M. BLOOM 3,756,868

METHOD OF ANNEALING STEEL COILS MOVING THROUGH A FURNACE Original Filed Feb. 26, 1969 5 Sheets-Sheet 1 From N Recondifioner INVENTOR WILLIAM M. 01.00

UM. Wat/Z1. 14

ATTORNEY Sept. 4, 1973 w. M. BLOOM 3,756,858

METHOD OF ANNEALING STEEL COILS MOVING THROUGH A FURNACE Original Filed Feb. 26, 1969 5 Sheets-Sheet 23 Fla. 2.

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INVENTOR WILLIAM u. 51.0041

ATTORNEY P 4, 1973 w. M. BLOOM 3,756,868

METHOD OF ANNEALING STEEL COILS MOVING THROUGH A FURNACE Original Filed Feb. 26, 1969 Sheets-Sheet 3 FIG. 6'. 76 FIG. 7.

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METHOD OF ANNEALING STEEL COILS MOVING THROUGH A FURNACE Original Filed Feb. 26, 1969 5 Sheets-Sheet 4 ,F/GZ 9!?" 3 1 l l I L T. T I04 I f 2 I I l X 88 II I !III/// INVENTOR 38 WILLIAM H. 01.0041

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@143 GM N h\\ m m m w w ATTORNEY United States Patent 3,756,868- METHOD OF ANNEALING STEEL COILS MOVING THROUGH A FURNACE William M. Bloom, Pittsburgh, Pa., assignor to Allegheny Ludlum Industries, Inc., Brackenridge, Pa. Original application Feb. 26, 1969, Ser. No. 802,548. Divided and this application May 4, 1971, Ser.

rm. c1. Htllf 1/04 US. Cl. 148-112 4 Claims ABSTRACT OF THE DISCLOSURE This application is a division of my copending application, Ser. No. 802,548, filed Feb. 26, 1969, now Pat. No. 3,062,289.

BACKGROUND OF THE INVENTION In the production of grain oriented magnetic alloys, such as silicon steels, a closely controlled processing anneal for grain orientation must be performed in order to provide a product of good magnetic and structural qualities. Annealing performed in a non-oxidizing atmosphere deters the magnetic alloys absorption of undesirable elements. Further, heating, soaking and cooling by controlled rates is necessary to assure production of material having desirable properties. Conventional equipment used for producing grain oriented silicon steel include high temperature batch furnaces and continuous type strip furnaces; however, each of these has disadvantages which are overcome by a semi-continuous tunnel-type furnace in accordance with the invention having a controlled atmosphere and which is capable of economically processing whole coils.

Batch furnaces are typically wasteful of heat energy because since the coils are heated, soaked and cooled in the same furnace components, the heating elements must have sufficient input energy to heat the coils and the furnace components themselves. Moreover, the higher heat input results in localized overheating in the outside wraps and edges of the coils which may result in deformation of the coil known as top edge flair and bottom edge oil canning. Additionally, all sections of the coils are not uniformly heated and variations in properties along the coil may occur. Because of the cyclic operation of batch furnaces, oxidants such as water droplets or carbon dioxide may appear on the metal surface and the resulting oxidation of the coiled strip surfaces increases surface emissivity which promotes differential heating of the coil. This also results in further coil deformation as well as an undesirable oxide anneal pattern on the surface of the strip. As with the heating cycle in the batch type processing, the cooling cycle also suffers from irregularities. The cooling of the furnace refractories during cooling of the coil in turn promotes differential cooling Within, which may also cause coil deformation and loss of magnetic properties. Additional disadvantages are incurred in batch processing through the atmosphere purge requirements. Production time is lost through the multiple changes of gases that are required in establishice ing a reducing atmosphere. Material is lost through the contamination and discharge of the nitrogen and hydrogen gases in purging the furnace in order to open the furnace for loading and unloading.

Some of the above disadvantages are overcome in a continuous process strip furnace. Separate zones may be established in such a furnace to regulate heating and cooling of the material as it proceeds through the furnace. Since each of the separate zones perform a continuous function, and are not temperature cyclic, the heat energy requirements are much lower. The continuous process also offers the advantages in that a continuous atmosphere may be maintained within the furnace. Time and gaseous matter are not lost in changing atmospheres. Heretofore, the continuous type annealing process has been applied only to strip processing wherein a coil of strip material was located at one end of the continuous furnace and the strip paid off of the coil and run through the furnace with the material recoiled after completion of the anneal. However, the nature of the heat treatment limits the efficiencies and economies possible by continuous strip processing. The present invention provides a furnace for effective coil annealing minimizing heat energy requirements and material deterioration during heat treatment while being capable of producing oriented silicon steel of uniformly high quality.

SUMMARY OF THE INVENTION The invention disclosed herein is a coil annealing furnace for annealing whole coils of steel on a continuous basis. To process whole coils, they are individually placed on cars and moved into the furnace through a vestibule from which they are pushed into and through a heating chamber comprising sections of increasingly higher temperatures to produce a predetermined controlled heating rate in the coils within a particular section of the furnace. A soaking chamber is provided which comprises sections of substantially the same controlled temperature. Also included is a cooling zone comprising sections of decreasingly lower temperatures to produce a predetermined controlled cooling rate for coils within a particular section. Within the furnace any section in the heating and soaking chambers may be used for heating or soaking except the first section which is for heating only and the last section which is for soaking only. Thus, a multiplicity of heating and soaking cycles can be obtained within the furnace to accommodate a variety of time and temperature cycles.

An integral part of the invention is the provision for a recirculated atmosphere system designed to maintain a clean, purified reducing furnace atmosphere relatively free of contaminants such as air, moisture, oxygen, carbon monoxide, carbon dioxide, hydrogen sulfide and nitrogen surrounding or in contact with the coil during the heating, soaking and cooling cycles. The recirculated gas is introduced into the furnace at the coil exit end of the cooling chamber and is caused to flow oppositeto the direction of coil travel through the furnace sections and through the heating chamber and the coil entry end where the atmosphere is removed, reconditioned, and recirculated. The cars upon which coils travel through the furnace sections are specially adapted for use within the furnace and have radiation tunnels located below the coil supporting surfaces which themselves are of special high conductivity material.

It is therefore an object of my invention to provide a heat treating furnace for processing whole coils on a continuous basis.

It is another object of the invention to provide a furnace suitable for annealing oriented silicon steel in coil form on a continuous basis.

A further object of my invention is to provide a coil annealing furnace which reduces coil damage which normally occurs during coil annealing due to non-uniform heating and cooling.

A still further object of my invention is to provide a coil annealing furnace which is of the recuperative type whereby a substantial portion of the heat applied to the coils to anneal same is recuperated or recovered and utilized to assist the uniform heating of coils not yet brought up to the annealing temperature.

Another object of my invention is to provide a coil annealing furnace which has a closed recirculating atmosphere which can provide coils with a reducing atmosphere immediately upon their entry into the furnace.

Still another object of my invention is to provide a coil annealing furnace which provides a closed recirculating atmosphere of a very low dew point to facilitate the removal of moisture from the coil strip surfaces immediately upon entry into the furnace.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) and FIG. 1(b) are broken sections showing the profile of the furnace including the heating, soaking and cooling chambers.

FIG. 2 is a partial sectional view of the initial heating and soaking sections taken at line IIII of FIGS. 1(a) and 1(b).

FIG. 3 is a partial sectional view of the final heat and final soak sections taken along line IIIIII.

FIG. 4 is a partial sectional view taken at line IV-IV showing the jack arch dividing the various heating, soaking sections.

FIG. 5 is a partial sectional view showing the initial cooling section taken along line VV.

FIG. 6 is a partial sectional view of the second cooling section taken along line VIVI.

FIG. 7 is a sectional view taken along line VII-VII of the third cooling section.

FIG. 8 is a partial sectional view of the fourth cooling section taken along line VIIIVIII.

FIG. 9 is an end view of coil supporting car including a partial section through the center of said car.

FIG. 10 is a sectional view of coil supporting car 40 taken along line (x) (x').

FIG. 11 is a schematic diagram showing the reconditioning system and nitrogen and hydrogen supply for the atmosphere of the furnace.

FIG. 12 is a chart of the internal coil temperature of the coil over the elapsed time within the heating, soaking and first cooling section of the furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in general and FIG. 1 in particular reference numeral 2 indicates a semi-continuous coil annealing furnace having an entry door 3 serving vestibule 4 located adjacent elevator shaft 5. Elevator shaft door 6 isolates vestibule 4 from shaft 5. Furnace entry door leads into furnace initial heating section 8. All doors are sealed with suitable gaskets 9. Continuing through furnace 2 on upper level 2' are initial soak section 10, final heat section 12, final soak section 14 and first cooling section 16 which terminates at door 18. Door 18, also sealed with gaskets 9, leads into elevator shaft 20 containing elevator 22, providing access to door 24 and second cooling section 26 on lower furnace level 2". Also in annealing furnace 2 are third cooling section 28, fourth cooling section 30 and fifth cooling section 31 which terminates at opening 32 providing access to elevator shaft 5. Shaft 5 contains an elevator 34 serving opening 32 and door 6 and door 7 providing access and exit from the furnace 2. Extending to and throughout furnace 2 are tracks 38 on which cars 40 carrying coils C traverse the furnace 2. Located adjacent elevator shaft 5 and aligned with heating section 8 is a hydraulic cylinder 42, such as A k r-liolth Hy raulic yl nder Seri s 100 4 Model 12C, having a rod 44 extending into shaft 4. Located adjacent shaft 20 and aligned with cooling sections 16 and cooling section 26 are similar hydraulic cylinders 42 having rams 44 extending into shaft 20. Within shafts 5 and 20, elevators 34 may be actuated by conventional systems such as a Vickers CHJO Custom Power Package.

Referring now to FIG. 2 which is a cross section of the initial heating section 8 and initial soaking section 10, reference numeral 50 indicates the gas tight welded steel tunnel shell through which cars 40 and coil C transit furnace 2. The tunnel has internal walls 51 and roof 53 lined with refractory brick 52 to a predetermined thickness, e.g. about 9 inches, on both sides and overhead and backed up further by insulation such as a second refractory 54, also of predetermined thickness, e.g. about 3" and a third refractory 56 such as vermiculite. Heating elements 58 which are preferably A diameter molybdenum rods available from Sylvania, Inc., are mounted on the walls 51 of tunnel 50. In the vicinity of car 40 the walls 51 have a built out portion 60 approaching car 40 and contain a slot 62 which cooperates with the protrusion 64 on car 40 to form a radiation shield between the coil area of tunnel shell 50 and the car understructure 66. Below the built out portion 60, the tunnel shell 50 is cut back 65 to the vicinity of the car understructure 66 and is unlined.

FIG. 3 represents a cross section of the final heat section 12 and final soaking section 14 of tunnel furnace 2 showing the additional layers of refractory that are contained within this section. The section structure is basically the same as that of the initial heating and soaking sections 8 and 10 shown in FIG. 2. Reference numeral 50 indicates the Welded steel tunnel shell having tunnel walls 51 with first refractory 52 and second refractory 54 and third refractory 56 of materials specified in the paragraph relating to FIG. 2. In addition to the foregoing structure a face refractory brick 52', e.g. about 11 in front of first refractory brick '52 and a backup refractory brick 54' such as 3" of thermobestos is placed along the interior of tunnel shell 50 between the shell and the previously mentioned second refractory 54.

Referring now to FIG. 4, reference numeral 70 in dicates modified jack arch lowering roof 53 and separating furnace sections 10 and 12 and sections 14 and 16 which provides an isolation between initial heating and soaking sections 8 and 10 and final heating and soaking sections 12 and 14 and first cooling section 16. The jack arch 70 is formed by laying additional courses 72 of refractory brick below the tunnel roof 53. The additional courses 72 are built down to limit tunnel area 50 so that it will just pass coil C as the coil transits the furnace 2. Similarly, walls 51 are extended outward to further isolate the adjacent sections by additional refractory brick 72.

FIG. 5 shows a cross section of first cooling section 16 of the furnace 2 having a gas-tight tunnel shell 50, walls 51 and roof 53 of which are formed with a first refractory brick 52 similar to preceding sections, a second refractory 54 and a third refractory 56 also similar to preceding sections. It is to be noted that the tunnel walls 51 in section 16 also include a slot 62 accommodating car protrusion 64 establishing the radiation shield and the shell 50 cut back 65 in the vicinity of the understructure 66 at FIG. 2. It is to be further noted that refractory thickness of walls 52 varies from section to section, the reasons for which will be described below in conjunction with furnace operation. Located on the center line of furnace 2 and at the center of the coil positions are cooling jets 74 connetced to piping 76.

FIG. 6 is a cross section of the second cooling section being reference numeral 26 shown on FIG. 1. The overall construction of the second cooling section 26 is similar to the first cooling section 16 described in FIG. 5 wherein a welded steel shell 50 surrounds and defines the furnace cavity which is lined with three refractory materials, a first refractory 52 which may be about 4" thick,

a second refractory material which may be about 3", and a third refractory material 56 such as 3" or more of vermiculite. Within the furnace roof 53 are cooling jets 74 connected to pipe 76 which directs the cooling gases into the center hole of coil C. The figure also shows a built out portion 60 in tunnel wall 51 including a slot 62 which cooperates with a protrusion 64 on car 40.

Referring now to FIG. 7 which shows a cross sectional view of third cooling section 28, reference numeral 50 indicates the welded steel wall of the furnace tunnel which is lined with a first refractory brick 52, e.g. about 4 /2" thick, and a refractory 56 such as 3" or more of vermiculite forming roof 53. As in previous figures showing sections of the furnace, there is a built out portion 60 having a slot 62 which cooperates with a similarly shaped structure 64 of car 40.

Fourth cooling section 34] is shown in FIG. 8 having a welded steel shell 51) lined with refractory material 56 such as about 3" suitable refractory on both side walls 52 and roof 53. A fifth cooling section 32 comprises an unlined steel shell 50.

Referring now to FIG. 9, reference numeral 40 indicates coil supporting car having understructure 66 including wheels 67, bearings 68 and axles 69 which support car upperstructure 78. Car upperstructure 78 includes a frame 80 supporting refractory material 82 to insulate the frame and the understructure 66. A second refractory material may be used for the additional protection against the higher temperatures as necessary. On the top of the refractory material are radiation tunnels 86 formed by Walls of a first quality fire brick, such as Ufala brick. These tunnels are located to cooperate in part with heating elements 58 located on tunnel walls 51. This is done so that a portion of the heat developed in the heating element 58 may be radiated into the tunnel to facilitate coil heating. Forming the top of the radiation tunnels are heating tiles 90 formed of a high conductivity, yet strong material such as Oxynitride-bonded silicon carbide plate as available from the Norton Company, Worcester, Mass. Providing a resting pad for coil C on the car structure 78 is hearth plate 92 of a low carbon steel having, for example a M coating of alumina. Hearth plate 92 contains a center hole 94 communicating with the center radiation tunnel 86. Also communicating with the center radiation tunnel 86 is a tube 95 carrying a thermocouple conductor 96 which extends to a plug in receptacle 97 in an area adjacent the car understructure 66. Within the refractory portion 82 of car 40 is a protrusion 64 which cooperates with slot 62 within the furnace tunnel wall 51 forming a radiation shield between portion of the tunnel containing the coil and the car understucture 66 area as previously described. This protrusion may be formed by stepping a course of the utilized refractory or a specially formed brick as shown in FIG. 9.

FIG. shows a side view of coil car 40, the trailing edge of which includes a slot 98 in the refractory portion. The leading edge of car 40 includes a protrusion 100 in the refractory portion 82. The slot 98 and the protrusion 100 are adapted to cooperate with similarly shaped protrusions and slots in adjacent cars. The cooperating slots and protrusions form radiation shields between the cars further isolating car understructure 66.

Detail of the atmosphere recirculation system is shown in the schematic of FIG. 11 wherein a recirculator 110 receives the contaminated atmosphere from furnace 2 entry end collected by a header 112 located on tunnel walls 51 above the level of the car hearth 9t) and transported through conduit 114. The recirculator preferably indicates several purifying and cooling devices to recondition the atmosphere for recirculation through the furnace, useful devices for the foregoing are V. D. Anderson Hi-EF Purifier, Model LBS-4-l0-304, a spiral heat exchanger type IV by the American Heat Reclaiming Corporation, an adjustable Roots-Connersville Rotary Gas Blower Type XA and an Engelhard Deoxo Tower Model D-3000-1 by the Engelhard Industries, Incorporated and a B-lSOO-SP Lectrodryer by the McGraw Edison Company. The reconditioner may also include a carbon monoxide to carbon dioxide conversion tower such as Engelhard Industries Selectoxo. The reconditioned atmosphere is returned to furnace 2 from recirculator through a conduit 116 to manifold 118 containing cooling jets 120. In the example disclosed there are five jets 120 located on the bottom of the tunnel at the coil exit end of furnace 2 adjacent elevator shaft 5 and ten on the sides of the sides of the tunnel 58 at the coil exit end, five of these jets being disposed on each of the two sides of the furnace. First cooling section 16 and second cooling section 26 are supplied additional hydrogen gas for cooling. The additional gas for cooling in the second cooling section 26 is collected at header 121 located in the lower side walls 51 above the level of the car hearth 96 in the furnace tunnel area 2" adjacent elevator shaft 20 and circulated through a cooler 122 such as a fin tube cooler available from Brown Fin Tube Company and then supplied through conduit 123 to manifold 76 which supplies cooling jets 74 located in the roof 53 of cooling section 26. The additional cooling gas for the first cooling section 16 is collected in a similar header 125 and supplied to a cooler 126 which in turn supplies a manifold '76 from conduit 127 supplying gas jets 74 also located on the external centerline of the roof 53 of first cooling section 16. The cooling jet 74 is located along the center line of the tunnel roof immediately above the center ofthe last group of coil stations on the upper furnace level 2' and above the first group of stations in the second cooling section 26 coil centerline 2". The atmosphere flowing through second cooling section 26 is conveyed to the first cooling section 16 by means of a collecting header 136 which is con nected to a conduit 132 and to a discharge manifold 134. Header is located adjacent the warmest or first coil in the second cooling section such that it will collect the atmosphere at a level above the car hearth 90 as it passes that coil and convey it to the first cooling section 16 discharging the gas through discharge manifold 134 located adjacent the side of the coolest or last coil in section 16.

Recirculator 110 is also connected to a hydrogen sup ply 138 through conduit 140 in order to provide makeup hydrogen for that lost through operational leakage of the system. A conduit 142 extends from recirculator 110 to a discharge manifold 144 located in vestibule 3 to provide hydrogen atmosphere to the vestibule prior to opening doors 6 or 7 to maintain the atmosphere within furnace 2. A nitrogen supply 146 is also connected to vestibule 4 through conduit 148 and discharge manifold 150 to provide a nitrogen purge of the vestibule to clear the hydrogen atmosphere from its prior to opening door 3 which allows air to enter vestibule 3. Headers 152 located high and low in vestibule 3 are vented to the outside to facilitate purging the existing atmosphere of vestibule 3 by a lighter (hydrogen) or heavier (nitrogen) gas to prepare the vestibule for opening to the furnace 2 or to the air environment outside the furnace 2. In the illustrated example the hydrogen and nitrogen discharge manifolds are equipped with suitable laminar flow nozzles to minimize mixing of the gases during purging. The nitrogen supply line 148 and the hydrogen supply line 144 include pressure regulators 154 which receive their pressure impulse from the vestibule internal pressure. The vestibule outlet lines have outlet fiow pressure regulators 154 with their pressure impulse from vestibule internal pressure.

FURNACE OPERATION The furnace 2 of this invention is constructed with a welded, gas-tight shell designed to maintain a clean, purified reducing atmosphere, such as hydrogen, within the furnace. A hydrogen gas-tight gasket 9 at doors 3, 6 and 7 and a purgible vestibule 4 allow charge and discharge of coils into the vestibule 4 and then into the elevator shaft 5 without any contamination of the furnace atmosphere. Coil C to be annealed is placed on a coil-supporting car 40 with the coil C resting on one of its ends, The cars 40 have a refractory 82 and 84 and steel frame 80 supported on wheels 67, bearings 68, and axle 69 and are moved along rails 38 into and through the furnace 2. The coil C in the example is silicon steel to be annealed for grain orientation are set on the hearth plate 92 which is supported by the hearth tiles 90 set over tunnels 86 which run laterally across the width of car 40. The car 40 containing coils C is pushed into the entry vestibule 4 through door 3 by any suitable means, such as a hydraulic cylinder ram. The outer door 3 is closed and the air in the vestibule 4 is purged out of the top of the vestibule through header 152 by nitrogen gas which enters the vestibule 4 through manifold 150 from the bottom of the vestibule 4. Upon completion of the air purge, hydrogen gas is then introduced into the top of the vestibule 40 to purge out the nitrogen gas through the vestibule bottom after which door 6 will be open into elevator shaft 5, the atmosphere of which is open to the furnace through opening 32. When the furnace door 6 is opened the car 40 and coil C may be pushed into the elevator shaft onto elevator 34 and raised to furnace level 2' by the elevator. When the car has reached the top furnace level 2', a hydraulic ram 44 powered by cylinder 42 pushes the car 40 and coil C into the furnace after door 7 has been opened, locating it at coil station 1 within the first heating section 8 of furnace 2. The door 7 is then closed and sealed against gasket 9 isolating the first heating section atmosphere from the elevator shaft. The cars proceed through the furnace 2 by being pushed to the new coil stations by subsequent cars being introduced into the furnace and assuming coil station 1. When there are cars in the furnace filling all of the coil stations on the upper furnace level 2', the introduction of subsequent cars into the furnace will require that cars be exited through door 18 into elevator shaft 20 onto elevator 22 after which door 18 is closed and sealed against its gasket 9. In the disclosed example elevator car 22 has an insulated hood 23 to surround the coil and car on three sides and the top so that the interior of the elevator shaft 20 is shielded from heat radiation from the coil. This shield further assists in maintaining constant cooling rates for the coils. When the car 40 is positioned within hood 23, the elevator 22 is lowered to furnace level 2". A hydraulic ram 44 and cylinder 42 push the car 40 and coil into second cooling section 26 through the opening at door 24 into the last coil position on that level. When the coil and car 40 are in position the ram 44 is retracted and door 24 is closed against gasket 9 isolating the atmosphere of the second cooling section 26 from the elevator shaft 20. The coils C are advanced through furnace level 2" as they were on furnace level 2 by being pushed into subsequent coil positions by additional cars being introduced into the second cooling section 26. When all of the coil stations in furnace level 2" are filled, the introduction of an additional car 40 and coil C into that deck will cause a car on coil to be exited through a doorless opening 32 into elevator shaft 5. That coil will be removed from the shaft through vestibule 4 via doors 6 and 3 after a purge of the vestibule 4 of hydrogen by nitrogen. To ensure a proper temperature history on the coils, each coil may have a thermocouple embedded in it which by leads 96 in tube 95 terminate in a plug-in receptacle 97 in the car under-structure. The furnace 2 has access doors (not shown) at each coil station to reach the receptacle 97 to attach leads by which external recording equipment (not shown) may read the temperatures.

RECIRCULATION ATMOSPHERE SYSTEM The atmosphere system includes circulation through the entire heating, soaking and cooling and sections of the furnace. Its purpose is to serve as a heat and contaminant transport to protect the steel coil as it is heated a d cooled. As the atmosphere is circulated through the furnace it picks up heat and contaminants from the coil surfaces and coil coatings and is withdrawn and cleaned. The flow of the atmosphere acts to fiush back toward the more contaminated coils, the off gases extracted by the dry, reducing atmosphere. In the flush back operation, the atmosphere containing some contaminants extracted from the hotter coils washes the relatively more contaminated, cooler coils enhancing the atmospheres ability as a transport of the off gases and minimizing its potential as a carrier of polutants. During the cleaning process the atmosphere is cooled and then is reintroduced into the furnace so that the only make-up gas required is for that which leaks out on the various seals on the furnace. The atmosphere is introduced into the furnace 2 on lower furnace level 2" at the discharge end through manifold 118 and jets 120 which are located on the bottom and the side walls 51 of the furnace. The atmosphere is circulated against the coil travel and toward elevator shaft 20 at predetermined rate such as about 30,000 s.c.f.h. Since a portion of the cool hydrogen is injected in the car understructure 66 area, it travels toward shaft 20 below the radiation shields 62-64 and 98-100 serving to cool that area.

This flow rate of the atmosphere provides adequate dryness in the atmosphere throughout the circulation and constant rate heating and cooling for the temperatures involved in the system in conjunction with heat input from the elements 58 and the heat dissipated through cooling jets 74 and through the cooling section walls. Changing the parameters of the system, such as the coil charge interval, rate of coil heating or cooling, temperatures, etc. could call for a change in rate of atmosphere flow. At the entry end of the lower furnace level, the gas (atmosphere) is collected with headers 130. It is transferred to the upper furnace level 2' and discharged into the first cooling section 16 at manifold 134. The gas continues through upper furnace level 2 against travel of the coils until reaching the entry end of furnace 2 where it is picked up by header 112 and returned to the recirculator 110 through conduit 114. As the relatively cooler hydrogen atmosphere is flushed back through the cooling sections, it settles around the successive coils above the hearth plate 92. The gradual heating of the hydrogen and cooling of the coils produces a smooth transfer of heat and the flow of atmosphere cools the coils at a constant rate. At the cross over point of lower level 2" of the furnace, a collecting header 121 collects a portion of the atmosphere circulating it through cooler 122 which reduces the temperature of the atmosphere from about 1700 F. to about 150 F., forwards it to manifold 123 and distributes it through to jets 74 located on the roof portion 53 of tunnel 50. These jets 74 are located above the center of the coil openings for a first group of coil stations along lower level 2" and direct a cooling jet down into the center of these cooling openings to provide additional cooling over that passed through the furnace refractory 52, 54 and 56 and by flow of the atmosphere to maintain the cooling rate set in the other cooling sections. The last group of coil stations of first cooling section 16 also are fitted with cooling jets 74 in the roof 53 of the tunneled portion 50 of upper level 2'. A portion of the atmosphere passing coil station 44 is collected in headers 125 and directed to a cooler 126 and further to a manifold 127 distributing the atmosphere to jets 74. The atmosphere being introduced into the furnace system at jets 120 is high purity hydrogen, e.g. 99.995% pure, at a temperature between and F. with a dew point of less than l00 F. As the atmosphere transits the lower level 2" of the furnace and reaches the entry portion of the second cooling section 26 its temperature has been elevated to approximately 1800 F. by carrying heat away from the coils C as it circulates past them. As the atmosphere flows through the upper level 21 of the furnace, the temperature is raised to 2150 F. at the soak section 14 and is retained at that temperature through this section. The eat gained area. The utilization of hydrogen as the atmosphere in the annealing furnace further promotes a cool understructure 66 in that the natural buoyance of the hydrogen concentrates the heat in the coil area of the tunnel as opposed to the area of the car understructure 66. Further, the cutback, unlined furnace structure dissipates much of the heat which does reach this area. Doors 7, 18 and 24 isolate upper furnace level 2' from lower furnace level 2" preventing a flow of heat from the coil area of level 2" to under car area of level 2'. The exchange of atmosphere from one level to the upper 2' is accomplished above the critical lower area by locating collecting header 130 and discharge manifold 134 adjacent the coils in the respective sections 26 and 16.

The material heat treated in the example disclosed is silicon steel strip in coil form which has been cold rolled and normalized and is to 14 gauge. Prior to being rolled into coils, the material is normalized with a bright shiny surface and electrolytically or slurry coated with a magnesium hydroxide coating and dryed. The refractory coating and air gaps between wraps of the coil combine to form an insulation against radial heat flow through the coil from the outer wrap to the inner center section. The furnace atmosphere is maintained reducing to the oxides on the wrapped surfaces of the coils to keep surface emissivity low and minimize the heat transfer radially through the coil wraps. The dew point is maintained low, e.g. below -15 F., in the hot section of the furnace to promote the removal of water vapor from within the wraps. However, silicon steel has been heat treated in the furnace with minimal oxidation due to the water vapor with dew points up to +10 F. Heat flow to and from the coils is promoted through the coil ends by high conductivity hearth plates supporting the coils and a flare cap on the exposed top edges of the coil. Heating of the coil wraps through the ends utilizing the high conductivity of the coil itself promotes the uniform heating of the coil by providing heat input to each wrap. By way of example, the vertical coil is supported by the high conductivity hearth plate and encapped with the flare cap and is heated at a rate of about 50 per hour. A lower heating rate could be chosen, however, this would lengthen the time required to raise the coil to the heat treating temperature. Lengthening the overall heating time would require either a longer heating chamber or a longer interval prior to the introduction of the additional coils into the furnace. The ability to promote the flow of heat into the coil limits the practical maximum heating rate allowable without coil deformation to about 100 F. per hour. The heating rate can be increased by laying the coil on the side and heating the ends of the coil by means of the heating elements 58 radiating directly on the ends of the coil. Such a method may produce a heating rate of as high as 150 F. per hour, however, some coil deformation may also be experienced. Further, it would be necessary to provide adequate support in the center of the coil to prevent the coil from collapsing as well as banding the outside diameter of the coil to prevent unraveling.

In the furnace described good grain growth and grain orientation is achieved by annealing at 2150 F. for a period of 21 hours in a dry atmosphere having a dew point of -15 F. or below and in atmosphere flow rate of 30,000 s.c.f. per hour. The heat treatment may be accomplished at lower temperatures such as 2000 F. over a longer period of time or the annealing temperatures may be increased to as high as 2250 F. for shorter periods of time. Operation with the parameters selected in the example minimize coil deformation and maximize coil yield by minimizing radiant heat input to the coils from the heating elements and by removing contaminants from the coil surfaces while the coils are still relatively cool and the strip material is less likely to react with the contaminants. The effectiveness of the reducing atmosphere maintained within the furnace as a means for suppressing radiant heat transfer to the coils is attested to by the bright shiny surface of the coil wraps exiting the furnace. The suppression of radiant heat transfer to the coil and the effectiveness of the conductive heating through the hearth plate is further confirmed by the minimum amount of coil deterioration through deformation as well as the uniform heating and cooling rate demonstrated in FIG. 12 showing the average coil temperature through the heating, soaking and first cooling sections of the furnace. Cooling of the coils is continued to a convenient temperature for removal from the furnace, such as F. in the example. Coils may be removed from the furnace at a higher temperature, up to approximately 800 F., however, removal at a temperature any higher than this could result in substantial oxidation of the surface of the coil and in deterioration of properties. The coils from the furnace herein disclosed are cooled at a continuous rate of approximately 40 F. per hour. So long as the coil is continually cooled, however, any rate conveniently obtain able within the cooling sections may be used. The combined effect of the various aspects of the invention is a furnace capable of grain growth and grain orientation annealing of full coils of silicon steel wherein the coils are heated at a more constant rate than previously known, maintains the coils at a constant temperature during soaking and then cools the coils at a rate better controlled than any previously known.

Although the furnace in the embodiment disclosed includes two levels, 2 and 2", one above the other, it could be constructed in one long chamber with an inlet and an outlet end and cooperating vestibules 4. The furnace could be constructed on a single level with the lines side by side utilizing a single vestibule 4 however eliminating the need for elevators such as 22 and 34. Since the furnace has a plurality of chambers for heating and also for cooling, heat treatments other than the grain growth and grain orienting anneal of silicon steels may also be performed. 'Heating and cooling rates can readily be controlled to establish predetermined desired values by controlling the input of heat by element 58 and the cooling of the charge by the atmosphere, rate of flow and by the input of cool atmosphere through jet 74 as well as controlled heat loss or dissipation through the Walls.

While one embodiment of my invention has been shown and described, it will be apparent that other adaptations and modifications may be made without departing from the scope thereof.

I claim:

1. A method of processing metal strip material in coil form which comprises introducing a plurality of coils one at a time into a scalable enclosure, purging said enclosure to provide a non-oxidizing atmosphere therein, passing each coil from said enclosure into a furnace structure to provide a row of coils therein, moving the coils through the furnace structure, heating the coils to heat treating temperature as they pass through the furnace structure and holding them at that temperature for a predetermined time, then cooling the heated coils, continuously introducing a cool purified non-oxidizing atmosphere into the furnace structure adjacent the cooled coils and circulating it back through the furnace structure countercurrent to the direction of movement of the coils therethrough, withdrawing said furnace atmosphere from the furnace structure adjacent said first-mentioned enclosure, reconditioning the withdrawn atmosphere to purify and cool it, utilizing the reconditioned atmosphere as the purified non-oxidizing atmosphere that is introduced into the furnace structure, passing each of said cooled coils in succession into a seal able enclosure, and Withdrawing the coils one at a time at periodic intervals from said last-mentioned enclosure.

2. A method according to claim 1, including purging said last-mentioned enclosure with a gas compatible with air every time is receives one of said coils.

3. A method according to claim 2, in which said nonoxidizing atmosphere is hydrogen.

by the atmosphere system flowing through cooling sections 30, 28, 26 and 16 is gained from the coils as the gas circulated around the coils. Heat is added to the system from the molybdenum heating elements 58 located along the walls 52 of tunnel selected sections. The heat is added to the furnace system to make up heat losses through the refractories of that section, all the while maintaining the temperature constant. In a constant temperature section, the atmosphere stabilizes the coil temperatures by transporting heat from any coil above the soak temperature to any coil below that soak temperature (2150 F. for example). Similarly, heat may be supplied to the atmosphere sections 12, and 8 by similar molybdenum heating elements 58 located along the tunnel walls 51 to supplement the heat given up to the coils. As the atmosphere transits to the first heat section 8 of the top level 2", its temperature is reduced from 2150 F. to approximately 1000 F. at header 112. This reduction in temperature of the atmosphere reflects the heat given up to the coils C within the heating sections 12, 10 and 8. Additionally heat is supplied to the atmosphere individually in sections 12, 1t} and 8 through molybdenum heating elements 58 located along the walls 52 of the tunnel 50 in such a manner to maintain the heat transfer rate to the coils equal throughout the sections.

In addition to the above recited means for adding to or removing heat from the atmosphere and coils within the atmosphere, temperature control in accordance with the invention is achieved by varying the amount of refractory material within the various furnace sections. It should be noted in FIGS. 2, 3, 5, 6, 7 and 8 that the cross sections of various sections of the furnace are varied. In FIG. 2, for example, the initial heating and soaking sections 8 and 10 of the furnace utilize relatively thick refractory materials making up the side walls 51. Comparing those heating sections with the final heating and soaking sections shown in FIG. 3 viz sections 12 and 14, wherein a great deal more heat is contained within the system it can be seen that the refractory walls in these sections are a great deal thicker. The total amount of refractory material in the section 12 and 14 walls amounts to approximately thick in an effort to retain as much heat as possible within the system and thus reduce the input requirements of the heating element 58. Beginning with cooling section 16 wherein it is desirable to dissipate some of the heat of the system and thus bring the temperature of the coils down to a convenient temperature for removal from the furnace, the thickness of the refractory walls decreases. In section 16 shown in FIG. 4 the total thickness is down to 15" and this area is provided with additional cooling from above by cooling jet 74. Proceeding to the second cooling section, section 26 on lower furnace level 2" shown in FIG. 6, it can be seen that the refractory wall thickness is about 10" and subsequent FIGS. 7 and 8 show that in successive sections 28 and 30 the refractory thickness decreases to about 7" and 3" respectively. In section 32 the heat is permitted to conduct directly through the tunnel shell 50. By varying the overall thickness of the furnace walls from section to section, particularly in the cooling areas of the furnace, the dissipation of heat from the interior of the furnace through the furnace refractory material to the surrounding atmosphere is controlled to assist in maintaining a constant cooling rate throughout the cooling sections 16, 26, 28 and 30 and 32. In the example described the heat loss in the cooling sections is maintained at an average of 400 B.t.u.s per square foot per hour which produces a F./hr. cooling rate in the steel coils throughout the initial heating and soaking sections. In final heating and soaking sections 8, 10, 12 and 14 the average heat loss is maintained at approximately 170 B.t.u.s per square foot per hour and the average coil heating rate of 43 F./hour is provided. The atmosphere exits the furnace at header 112 and is returned to the recirculator 110 laden with the contaminants from the coil surfaces such as hydrogen sulfide, water, carbon dioxide, carbon monoxide, oxygen and nitrogen removed in the coils. Within recirculator 110 the atmosphere passes through the spin-type drying filter previously mentioned where all the dust particles larger than 10 microns fall out by impingement and centrifugal action. Also within the recirculator 110 the gas enters the spiral type water-cooled heat exchanger at 600 F. to be cooled to to F. the temperature at which it is returned to the furnace system. Within the preferred embodiment disclosed herein, the gas is then circulated through a series of bag-type dry filters within the recirculator which remove all dust particles /2 micron and larger from the atmosphere system. The oxygen, hydrogen, sulfide, air, carbon dioxide, carbon monoxide and water vapor are then removed in the Deoxo Tower and the Lectrodryer restoring the gas to its purity of 99.5% and dew point of below about -100 F. The gas is then returned to the furnace system through jets 120.

In the operation of the purging systems within purging vestibule 4 the natural density of the various gases is utilized to reduce the quantity of the purged gas required. Gases of lighter weight such as hydrogen are put into the top of vestibule 4 to purge heavier gases out the bottom such as air or nitrogen, which exit through headers 152 to bottom of vestibule 4. Conversely, heavier gas, such as nitrogen, is put into the bottom of the vestibule through manifold 150 to purge lighter gas out of the top of the vestibule through a second header 152" located at the top of vestibule 4-. As in the example, the gases may be injected into vestibule 4 through a distribution manifold having laminar floor nozzles to minimize mixing of the purging gas with the purged gas. Further, if as in the example, the supply flow rates are equated to the exhaust flow rates, excessively high or low pressures within the vestibule 4 are eliminated during the purge and leakage past the gaskets 9 is minimized. A purging chamber such as the vestibule 4 having laminar flow nozzles and flow rate control of supply and exhaust provides a complete purge with a volume purging gas equal to twice the volume of the chamber. Conventional systems require five to eight chamber volumes of purging gas.

Uniform heat input to the top edge of coil is promoted through the use of a flare cap 102. This flare cap covers the corner of the coil around the outer wrap to suppress radiation heat transferred to the outer top corner of the coil. A center mandrel 104 inserted in the center of the coil C juxtaposed with the inner wrap and suppresses radiation transfer to this portion of the coil and provides structural support. This protection against radiant heat transfer suppresses deformation of those exposed areas of the coil and prevents unraveling of the coil wraps.

The car understructure 66 displayed in FIGS. 5 and 6 is kept at a relatively low temperature through various means. The low temperature in this car understructure area is very important in that the lubricants used normally have a very high carbon content and the tendency at high temperatures would be for the carbon to be volatilized into the atmosphere and could be absorbed by the coils. Absorption of carbon by the silicon steel should be suppressed because of both the aging which occurs from the inclusion of the additional carbon, and also because of the effect on the magnetic properties of the material by virtue of the carbon occupying intersticial spaces in the crystal structure of the steel. Means to keep the car understructure 66 cool include refractory shielding accomplished by refractory materials 82 and 84 as well as radiation shielding accomplished by the protrusions and slots in the car structure 98 and 100 and protrusion 64 in the car side structure cooperating with slots 62 in the tunnel walls 51. Further promoting the lower temperatures in the car understructure region is the fact that the atmosphere in the furnace 2 is always collected in the coil or upper tunnel area as opposed to the car understructure area. Some of the main atmosphere supply jets are located below the cars 40 thereby also promoting cooling in this 3 14 4. A method according to claim 3, in which said hy- 3,640,780 2/1972 Stanley 148-113 drogen is introduced into the furnace structure at a dew 3,606,289 9/1971 Bloom 148113 point not greater than -60 F.

References Cited UNITED STATES PATENTS U.S.C1.X.R. 3,409,480 11/1968 Forslund 14s-113 3,345,219 10/1967 Detert 14s-112 148*421'155 1,752,490 4/1930 Karcher 14s--112 L. DEWAYNE RUTLEDGE, Primary Examiner 5 W. R. SATIERFIELD, Assistant Examiner 

